Two starting batteries. The boat would not start using battery 2. Battery 2 registered 1.1 V and so I replaced it although it was only 2 1/2 years old. Two weeks later the boat would not start using battery 2 again. Voltage measured 2.2 V. After running the boat for onlly 5 minutes, battey 2 now measured 12.5 V. The hookup for battery 2 is simple. One main cable going directly to the main battery switch and one very small cable to the Mercury cathode system. The only thing I can think of is that downstream there is some sort of short circuit draining battery 2.
It is even more complicated than this, but I primarily need some ideas of what to do next.